snowflake-01: Verify fallback boot partition(s)

In #40497 (closed) snowflake-01 was reinstalled on the SSD's, taking the failing SuperDOM devices out of commission.

In an attempt to make the boot process and alternative boot partitions more predictable, we're taking software raid (md(4)) out of the equation by installing GRUB directly to the first partition of both SSD's and not adding them to an MD array.

This issue is about verifying that the machine can indeed boot from all boot partitions.